1. Technical Field
The present invention relates to photolithography, and more particularly, to a photolithography technique and apparatus that is capable of.
2. Description of the Related Art
The following papers provide useful background information, for which they are incorporated herein by reference in their entirety, and are selectively referred to in the remainder of this disclosure by their accompanying reference numbers in triangular brackets. For example <1> refers to the 2006 paper by Thompson.    1. S. E. Thompson, S. Parthasarathy, Mater. Today 9, 20 (2006).    2. H. Ito, in Microlithography—Molecular Imprinting. (Springer-Verlag Berlin, Berlin, 2005), vol. 172, pp. 37-245.    3. S. Kawata, H. B. Sun, T. Tanaka, K. Takada, Nature 412, 697 (2001).    4. M. Rumi, S. Barlow, J. Wang, J. W. Perry, S. R. Marder, in Photoresponsive Polymers I. (2008), vol. 213, pp. 1-95.    5. C. N. LaFratta, J. T. Fourkas, T. Baldacchini, R. A. Farrer, Angew. Chem. Int. Ed. 46, 6238 (2007).    6. D. Yang, S. J. Jhaveri, C. K. Ober, Mater. Res. Sci. Bull. 30, 976 (2005).    7. J.-F. Xing et al., Appl. Phys. Lett. 90, 131106 (2007).    8. D. Tan et al., Appl. Phys. Lett. 90, 071106 (2007).    9. W. Haske et al., Opt. Express 15, 3426 (2007).    10. H.-B. Sun, T. Tanaka, S. Kawata, Appl. Phys. Lett. 80, 3673 (2002).    11. T. A. Klar, S. Jakobs, M. Dyba, A. Egner, S. W. Hell, Proc. Nat. Acad. Sci. USA 97, 8206 (2000).    12. S. W. Hell, Science 316, 1153 (2007).    13. S. W. Hell, Nat. Methods 6, 24 (2009).    14. C. S. Colley et al., J. Amer. Chem. Soc. 124, 14952 (2002).    15. J. O. Hirschfelder, C. F. Curtiss, R. B. Bird, Molecular Theory of Gases and Liquids (Wiley, New York, 1954), p. 890.    16. L. Li et al.,“Achieving λ/20 Resolution by One-Color Initiation and Deactivation of Polymerization” Science, released on Science Express, Apr. 9 2009.    17. M. W. Beijersbergen, R. P. C. Coerwinkel, M. Kristensen, J. P. Woerdman, Opt. Commun. 112, 321 (1994).    18. B. Harke, C. K. Ullal, J. Keller, S. W. Hell, Nano Lett. 8, 1309 (2008).    19. R. A. Farrer, F. L. Butterfield, V. W. Chen, J. T. Fourkas, Nano Lett. 5, 1139 (2005).
<16> specifically is a paper related to this disclosure and provides additional information that may have been omitted from this disclosure for conciseness.
The demand for increasingly powerful integrated circuits has spurred remarkable progress in lithographic techniques in the past decades <1>. However, progress towards higher resolution has proven to be increasingly difficult and expensive as feature sizes decrease. To improve resolution in photolithography, chemical nonlinearity can be employed to create a sharp intensity threshold for exposure <2>. However, diffractive effects still limit feature sizes in conventional photolithography to approximately a quarter of a wavelength (λ) of the light used to expose the photoresist.
Nonlinear-optical phenomena provide an alternative approach to photolithography <3-6>. In multiphoton absorption polymerization (MAP), a photoinitiator in a prepolymer resin is excited by the simultaneous absorption of two or more photons of light. The absorption probability depends upon the laser intensity to the power of the number of photons needed to cause an electronic transition, and so an ultra fast laser is generally used to provide high peak intensity at low average power. The laser is focused through a microscope objective such that the intensity of the light is only high enough to drive multiphoton absorption in the small region defined by the focal volume of the beam. In the most common implementation of MAP, multiphoton absorption initiates cross linking that hardens the prepolymer resin within the focal volume. Once you excite the photoinitiator, it drives a chain reaction that leads to the polymerization of the prepolymer resin. This polymerization can be confined to a focal volume using a focusing instrument, as discussed above. By moving this focal volume relative to the sample, complex, 3-dimensional polymeric structures can be created.
Due to the optical nonlinearity of multiphoton absorption and the existence of an intensity threshold for polymerization, MAP can be used to create volume elements (voxels) with a resolution that is considerably smaller than the wavelength of the light used. For instance, 800 nm light has been used with MAP to create voxels with a transverse dimension of 80 nm <7>, corresponding to λ/10 resolution. Even finer resolution has been reported for suspended lines, although based on the tapered nature of these lines at their attachment points it is likely that shrinkage during the developing stage plays a role in this case <8>. Using light of a shorter wavelength for MAP can also improve resolution <9>. It should be noted that due to the shape of the focal region of the laser beam, the resolution of MAP along the beam axis is usually a factor of at least three poorer than the transverse resolution <10>.
In optical fluorescence microscopy, extraordinary resolution has been achieved using a technique called stimulated emission depletion (STED) <11-13>. In STED, fluorescent molecules are excited by a short laser pulse. A second laser pulse, which is tuned to a significantly longer wavelength than the first pulse, is used to de-excite the molecules through stimulated emission. This depletion pulse must arrive after vibrational relaxation is complete in the excited electronic state but before significant fluorescence has occurred. Spatial phase shaping of the depletion beam causes de-excitation to occur everywhere except in a chosen region of the original focal volume <11-13>. This chosen region is where the fluorescence takes place and hence, fluorescence can be localized in a zone much smaller than the excitation wavelength. The size of this region depends on the intensity of the depletion beam and the corresponding degree of saturation of stimulated emission.
A potentially powerful extension of STED is to employ it for photolithography. A number of groups around the world are attempting to implement STED photolithography. However, to the knowledge of the inventors of this disclosure, no one has yet been successful.